| Non-aqueous lithium-air batteries have great prospects in the fields of electric vehicles,mobile energy and aerospace due to their ultra-high theoretical energy density(3458 Wh?kg-1).However,lithium-air batteries are still in the initial stage and exist many problems,such as high over-potentials,poor cycling life,slow oxygen reaction kinetics and lithium dendrites,which further limits their practical application.In this paper,two methods,including introduction of electrolyte additive and modification of carbon materials were proposed to vest a better electrochemical performance for lithium-air battery.For electrolyte additives,we use potassium iodide(KI)as a bifunctional additive to reduce the charge overpotential and improve both the reactivity of cathode and the stability of lithium anode,cycling performance of lithium-air batteries with limited capacity of 500 m Ah?g-1 is increased by 2 times.On the one hand,the reversible redox reaction ensures I-acts as "redox mediator" in electrolyte,which can reduce the charge overpotential by 0.7 V.From SEM?FTIR and other results,it can be seen that I-promotes the decomposition of discharge products and suppresses the attenuation of electrochemical performance in cathode.On the other hand,the introduction of potassium ion(KTFSI)is found to lower voltage difference around 50 m V in Li|Li symmetry cells and increase its cycling life.Also,the results in lithium-air batteries demonstrate that K+ contributes to inhibiting the growth of lithium dendrites,optimizing the morphology,reducing surface by-products and thus improving electrochemical performance of lithium anode.Moreover,K+ also helps to promote the solubility of superoxide(Li O2),which formed during discharge process,and facilitate the solution-route formation of Li2O2.After adding K+,the discharge capacity of lithium-air batteries increased from 3100 m Ah?g-1 to 6100 m Ah?g-1 at a discharge current of 100 m A?g-1.By using additive KI,a more excellent cycling performance of lithium-air batteries can obtain comparing with Li I additive.Further combining I-with catalyst 2,5-di-tertbutyl-1,4-benzoquinone(DBBQ)in electrolyte,a significant improvement of cycling stability can realize even with a higher capacity.In terms of modification of carbon materials,commercial BP2000 is used as the cathode,and a small amount of selenium element is doped into the carbon material through high-temperature calcination with diphenyl diselenium(DDS)as selenium source.The Se-doped carbon material reveals a best discharge performance in lithium-air batteries when the mass ratio of carbon to selenium source is 2:1 and the calcination temperature is 1000 °C.Physical properties of carbon material are analyzed by using EDS,Raman,XPS and other technologies,the results demonstrate that selenium atoms can successfully doped into the carbon material to formed C-Se bond,producing more defects and active sites,which are beneficial to the improvement of oxygen reduction reaction as shown in RDE test.Battery performance of different carbon materials in lithium-air batteries show that the discharge capacity was increased from 2635 m Ah?g-1 to 3868 m Ah?g-1 at a current density of 100 m A?g-1 after doping selenium element.Which also decreases the polarization in discharge about 40 m V and improves cycling performance of the battery.Mechanism of Se-doped material improving battery performance are explored through analyzing the morphology and composition of discharge products,and the results show that Se-doped material helps to improve the activity of oxygen reduction reaction,and promotes solution-route formation of discharge products,thus leading to a more uniform and dense distribution of toroid-like products.At the same time,some byproducts like Li2CO3 are obviously reduced,thereby boosting the performance of lithiumair batteries. |
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